In large scale integrated circuits (LSI) or micro electro mechanical systems (MEMS), a device preparation process is carried out by repeating formation of a thin film and formation of a pattern. Such a pattern is a resist pattern formed via exposure, development and washing (rinsing) or an etching pattern formed via etching and washing.
A resist to be used in the device preparation process is a polar polymer material sensitive to light, X-ray, electron ray, etc., and will be removed after etching. The resist remains at a pattern portion even after development or etching treatment, and the resist is in a state embedded in pore portions of a pattern. Further, in a case where a low dielectric constant film is formed on the surface of a wafer, and a resist is applied thereon, the resist may sometimes penetrate into fine pores of the low dielectric constant film. Therefore, in washing a device substrate, it is necessary to remove a resist penetrated into such fine pores of a pattern or fine pores of a low dielectric constant film.
In washing to carry out removal of such a resist, an acid solution composed mainly of sulfuric acid or the like, has been used as a wash solution. However, such a wash solution has a large surface tension and can not be penetrated into fine pores of a pattern having a large aspect ratio (height/width), whereby there has been a problem that a resist remains in the pores of a pattern even after the washing.
On the other hand, an ashing technique by means of an oxygen gas plasma has become a prospective means to remove a resist in recent years, since it is free from a problem of a waste liquid, and the gas will diffuse into the interior of a fine structure.
However, in a case where a pattern to be formed is made of an organic material, it is difficult to employ such an ashing technique for removal of a resist.
For example, in LSI, as a material to be used for an interlayer insulating layer of a multilayer wiring structure, a low dielectric material such as a porous methylpolysiloxane or a hydrated polysiloxane has been used to lower the wiring capacity along with the high speed of a semiconductor devices. However, if such an interlayer insulating layer is masked with a resist pattern and subjected to etching and then ashing is carried out to remove the resist pattern, there is a problem such that methyl groups or hydrogen constituting the interlayer insulating film will also be removed by ashing, whereby the dielectric constant will increase.
As a method for solving such a problem, a washing method has been proposed wherein a supercritical fluid having both a gas diffusion property and a liquid-dissolving property is used as a wash solution (e.g. Patent Document 1). By such treatment employing a supercritical fluid, carbon dioxide has heretofore been used as the supercritical fluid.
In the above-mentioned treatment employing a supercritical fluid, a substrate having a resist or the like remaining thereon, is placed in a high pressure resistant container, and carbon dioxide in a supercritical state is introduced into such a high pressure resistant container, so that carbon dioxide in a supercritical state will act on the substrate. The supercritical fluid has a small surface tension and a large diffusion coefficient, whereby it can easily be penetrated even in fine spaces and thus has a possibility of capable of removing a resist between patterns.
However, carbon dioxide does not have a dipole moment, and a resist made of polar molecules can not substantially be dissolved therein. Thus, by the conventional method employing a supercritical fluid, it was not possible to sufficiently remove the resist.
The following two methods have been proposed as methods to solve the above-mentioned problem in the washing method employing a supercritical fluid.
The first method is a method wherein a wash solution is used to dissolve the resist, followed by replacing it with carbon dioxide in a supercritical state (Patent Documents 2 and 3).
The second method is a method wherein an assisting agent such as an amine, an alcohol or glycol ether is employed in carbon dioxide (Patent Document 4).
Further, Patent Document 5 discloses, as a process for carrying out supercritical treatment at a lower pressure, “a supercritical treating process comprising at least a first step of bringing a prescribed pattern on a substrate in a state immersed in a liquid of a fluoride under the atmospheric pressure, a second step of disposing the substrate in a space sealed and maintained to have a constant volume in a state where the above pattern is immersed in the above liquid, a third step of heating the liquid in which the above pattern is immersed to bring the above liquid to a supercritical state and to make the above pattern in such a state as immersed in the supercritical fluid of the above fluoride, and a fourth step of evaporating the supercritical fluid in which the above pattern is immersed, wherein in the third step, the pressure in the above space is brought to the critical pressure of the fluoride by an increase of a gas of the fluoride heated and vaporized”. It is disclosed that the fluoride to be used for this supercritical treating method is preferably at least one of a hydrofluoroether and a hydrofluoroester.
On the other hand, the present inventors have previously proposed an azeotropic solvent composition comprising 89 mass % of 1,1,2,2-tetrafluoro-1-(2,2,2-trifluoroethoxy)ethane and 11 mass % of 2,2,2-trifluoroethanol, as a novel solvent composition which is non-flammable, excellent in a dissolving power and useful for a wide range of applications (Patent Document 6).    Patent Document 1: JP-A-08-181050    Patent Document 2: JP-A-10-260537    Patent Document 3: JP-A-2003-206497    Patent Document 4: JP-A-2003-513342    Patent Document 5: JP-A-2006-40969    Patent Document 6: JP-A-2004-75910